The assignee of the present invention, The Brunton Company of Riverton, Wy., has for the past several decades manufactured and sold portable transits incorporating a magnetic compass in which the needle and magnet assembly were mounted on a jewel bearing not only to provide precise compass readings but also to operate in combination with a pair of sights and a graduated scale for measuring angles and grades. The Brunton transit possesses a number of other features, such as, for instance, induction damping which will allow the needle to seek magnetic north and come to a complete rest in an extremely short time. Induction damping is achieved through the utilization of a damper pan upon which the needle and magnet assembly rests but, as opposed to working on the principle of friction, is a velocity dependent force, i.e., as the speed of oscillation of the needle decreases, the influence of this type of damping force also decreases so that when the needle comes to rest the influence of the induction damper is zero. The reading obtained in this manner is accurate and much quicker than one obtained with an undampened instrument.
Magnetic declination adjustment is afforded in the Brunton pocket transit in order to compensate for changes in latitude when the compass is shipped to different parts of the world. In other words, magnetic declination, that angle between magnetic north as indicated by a compass and true north, in the past has been compensated for by rotation of the graduated scale a number of degrees corresponding to the declination. Still further, by virtue of the relationship between the sight assembly, compass and vernier adjustment associated with the body of the compass, the standard pocket transit is capable of measuring various horizontal angles or courses as well as vertical angles or inclines. In the pocket transit the vernier is manually adjustable through the bottom wall of the casing and operates in cooperation with another graduated scale for measuring grade in percentages.
Generally, the magnetic vector which acts upon the needle and magnet assembly can be resolved into two force components: The horizontal component parallel to the earth's surface, and the vertical component perpendicular to the earth's surface. The horizontal component forces the permanent magnet needle of the compass to point to the north magnetic pole while the vertical component causes the needle to be tipped away from the horizontal position. In order to compensate for this effect, the vertical component of force which tends to force one end of the needle up and the other end down is balanced out by the addition of weight to the opposite end of the needle which is tipped down thus compensating for the vertical component of force causing the needle to tip and is very time consuming in the original manufacture of the compass. Moreover, since the strength of the magnetic vector components varies around the world, it is necessary to rebalance the needle at each new location, especially at locations far removed from the initial balancing point. This poses substantial problems with respect to compasses which are sealed and cannot be rebalanced without breaking the seal; also where users are not equipped to rebalance the needle or don't recognize the problem it is necessary to return the unit to the manufacturer for rebalancing. In the past it has been proposed to displace the pivot point for the needle above the permanent magnet and for example reference is made to U.S. Pat. Nos. 3,191,306 to Kiernans and 2,552,332 to LeVan. LeVan in particular discloses a graduated scale and magnet assembly in which the scale is mounted on a pivot point displaced above the magnet assembly so that in any latitude the tilt of the moving parts, i.e., the declination due to the vertical magnetic field will be minimized; and a jewel post assembly is employed with a spring-loaded stem supported in a damping cup in order to minimize damage to the surface of the jewel or pivot. To my knowledge, however, a magnetic compass assembly has not been devised which is conformable for various different applications as a compass or transit as described but which at the same time provides a needle and magnet assembly which can be operated world wide, without having to rebalance the needle at each different location; yet which will provide a rugged but sensitive and accurate needle suspension system with induction damping, obviate the need for a V-type jewel bearing and lift system for the needle support, and permit compact mounting within a minimum of space.